Since their invention, glass micropipettes have been indispensable tools for molecular and cellular biologists. They can be used for microinjection to mechanically penetrate the cell's membrane and inject or sample material to and from the cell's interior. Examples include transplanting the nucleus, making molecular cloning possible. Micropipettes can also be used to make electrical contact to the cell interior, which is the foundation of the patch-clamp technique.
Glass micropipettes are suited for establishing a mechanical contact with a single cell by penetrating through the lipid membrane. However, penetration may often be unfavorable, since it can destroy or deteriorate the internal structure of the cell and affect its normal physiology. Thus, in many experiments it is favorable to communicate with the cells in the same way nature does—by external chemical stimulation. However, conventional glass pipettes are difficult to use in order to establish a non-mechanical, purely chemical contact with a microscopic object. For example, if exposure of the cell to a chemical stimulus is desired, a conventional micropipette can be used to inject the active substrate to the vicinity of the cell. To maintain constant concentration, such an injection must be continuous; otherwise the concentration will decrease rapidly by diffusion. The constant injection on the other hand will cause accumulation of active substrate outside of the microscopic experimental region, which contaminates other cells or surfaces and severely limits further studies in the same system.
Therefore, there is a need in the art for a device that is capable of suitably controlling the chemical environment in the microscopic region around the cell or around an artificial sensor element, without cross-contaminating other cells in the culture or other elements fabricated on the same surface.
Further, many experiments require that the object of interest is exposed to a programmed sequence of chemical stimuli, with well-controlled temporal resolution. This can be achieved by different superfusion techniques, which can be based on different glass tubes, for example, the θ-tube or multi-barrel systems, where solution exchange times can be as short as 200-400 μs. However, these devices may result in cross-contamination and do not allow spatial concentration control with high precision.
Accordingly, to address such high-resolution spatial concentration control, microfluidics systems can preferably be used. However, conventional microfluidic devices lack an important capability of a pipette, which is the control of position relative to the macroscopic reservoir, or open volume. Instead, the microscopic sensor (for example, but not only limited to a single cell) must be scanned in front of the channel array for superfusion. This considerably restricts the applicability of the superfusion device and makes it considerably more difficult to expose an arbitrary region such as a cell, microfabricated structures, sensors, and/or actuators attached on a surface, a single cell in a large cell culture or tissue, extended structures built on a surface (e.g., lipid-nanotube networks), and micro/nanofabricated objects on a surface.
Further, some experiments require the exposure of microscopic objects to precisely defined chemical gradients. For example, studies of chemotaxis are important for the understanding of embryogenesis, cancer metastasis, cell growth, and tissue formation. Stable microscopic chemical gradients are difficult to achieve with macroscopic tools. Currently, microfluidics have been employed for these studies. However, cell handling in microfluidic devices is not trivial and it is highly desirable to generate chemical gradients on arbitrary areas in an open reservoir, where an object of interest is located, either as a single sensor or within a collective of sensors.
In addition, various mechanical and practical problems may be associated with glass pipettes, such as backfilling, breaking upon accidental contact with surfaces or objects, damage to cells and tissue, adsorption of chemical agents or biological matter to the glass surface, the need for specialized pulling equipment, and the need for a specifically designed needle holder.
Accordingly, there remains a need in the art for improved microfluidic pipettes.